Methods for Controlling Warpage in Packaging

ABSTRACT

A method includes placing a plurality of dummy dies over a carrier, placing a plurality of device dies over the carrier, molding the plurality of dummy dies and the plurality of device dies in a molding compound, forming redistribution line over and electrically coupled to the device dies, and performing a die-saw to separate the device dies and the molding compound into a plurality of packages.

BACKGROUND

With the evolving of semiconductor technologies, semiconductorchips/dies are becoming increasingly smaller. In the meantime, morefunctions need to be integrated into the semiconductor dies.Accordingly, the semiconductor dies need to have increasingly greaternumbers of I/O pads packed into smaller areas, and the density of theI/O pads rises quickly with time. As a result, the packaging of thesemiconductor dies becomes more difficult, which adversely affects theyield of the packaging.

Conventional package technologies can be divided into two categories. Inthe first category, dies on a wafer are packaged before they are sawed.This packaging technology has some advantageous features, such as agreater throughput and a lower cost. Further, less underfill or moldingcompound is needed. However, this packaging technology also suffers fromdrawbacks. As aforementioned, the sizes of the dies are becomingincreasingly smaller, and the respective packages can only be fan-intype packages, in which the I/O pads of each die are limited to a regiondirectly over the surface of the respective die. With the limited areasof the dies, the number of the I/O pads is limited due to the limitationof the pitch of the I/O pads. If the pitch of the pads is to bedecreased, solder bridges may occur. Additionally, under the fixedball-size requirement, solder balls must have a certain size, which inturn limits the number of solder balls that can be packed on the surfaceof a die.

In the other category of packaging, dies are sawed from wafers beforethey are packaged, and only “known-good-dies” are packaged. Anadvantageous feature of this packaging technology is the possibility offorming fan-out packages, which means the I/O pads on a die can beredistributed to a greater area than the die, and hence the number ofI/O pads packed on the surfaces of the dies can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1A and 1B illustrate a top view and a cross-sectional view,respectively, of an intermediate stage in the manufacturing of a fan-outpackage in accordance with some exemplary embodiments, wherein devicedies and dummy dies are placed over a carrier;

FIGS. 2-4 illustrate the intermediate stages in the formation of thestructure shown in FIGS. 1A and 1B;

FIGS. 5 and 6 illustrate the formation of redistribution lines toconnect to the device dies, and the die-saw process to separate packagesfrom each other; and

FIGS. 7 through 13 illustrate the intermediate stages for placing dummydies over a carrier in accordance with alternative embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A Fan-out wafer level package and the method of forming the same areprovided in accordance with various exemplary embodiments. Theintermediate stages of forming the package are illustrated. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1A and 1B illustrate a top view and a cross-sectional view,respectively, of an intermediate stage in the manufacturing of apackage. Referring to FIG. 1B, carrier 20 is provided. In some exemplaryembodiments, carrier 20 is a blanket wafer, which may be a glass wafer,a quartz wafer, a ceramic wafer, an organic wafer, or the like.Furthermore, carrier 20 may be formed of a homogeneous material. Carrierwafer 20 has a planar top surface. Carrier 20 may have a circulartop-view shape, as shown in FIG. 1A. Die-attach film 22 is laminated onthe planar top surface of carrier 20. In some embodiments, die-attachfilm 22 comprises an organic material, which may be a polymer-basedmaterial. Die-attach film 22 may be adhesive when heated.

Device dies 24 and dummy dies 30 are placed over, and adhered to,die-attach film 22. In some embodiments, device dies 24 includesemiconductor substrates 26 (FIG. 5), and active devices such astransistors 28 formed at the surface of semiconductor substrates 26.Semiconductor substrates 26 may be silicon substrates, or may be formedof other semiconductor materials. In the illustrated embodiments, thereare two device dies 24 placed next each other as a group, which group issawed in the subsequent step into the same package 60 (FIG. 6). The twodevice dies 24 in the same package may have different structures,although their structures may also be the same as each other. Inalternative embodiments, one resulting package 60 (FIG. 6) may includeone device die or more than two device dies.

As shown in FIG. 1A, device dies 24 are placed to overlap the inner areaof carrier 20, and are placed as a plurality of rows and columns,wherein each row has a width equal to W1, and each column has a lengthequal to L1. Each of the areas in one row and one column is referred toas a package area, in which a package will be formed.

In the peripheral area of the carrier, there are some areas that are notlarge enough to place device dies 24. These chip areas (referred to aspartial chip areas) are in the peripheral region 20A of carrier 20,which peripheral region 20A extends from the edge of carrier 20 towardthe center of carrier 20. Peripheral region 20A encircles inner region20B. In some embodiments, the peripheral region includes a ring that haswidth W2 smaller than about 25 mm. Dummy dies 30 are placed in thepartial chip areas. In some embodiments, no dummy die 30 is placed tooverlap inner region 20B of carrier 20. Throughout the description, theterm “dummy die” refers to the die or chip that does not have anyelectrical function, and the dummy die does not contribute to theelectrical operation of the resulting package. Dummy dies 30 may beformed of a homogenous material without any circuit, metal line, and/orsub-layer therein.

Dummy dies 30 may be formed of glass, quartz, blank silicon, or thelike. The Coefficient of Thermal Expansion (CTE) of dummy dies 30 islower than the CTE of molding compound 36 (FIG. 1B), which are moldedbetween device dies 24 and dummy dies 30. In some exemplary embodiments,the CTE of dummy dies 30 is between about 0.5 ppm/° C. (μm/m/° C.) andabout 7.5 ppm/° C. It is appreciated, however, that the values recitedthroughout the description are merely examples, and may be changed todifferent values.

In some embodiments, as shown in FIG. 1B, the thickness T1 of dummy dies30 is close to the thicknesses T2 of device dies 24, for example, withthe difference between T1 and T2 being smaller than about 10 percent ofeither one of thicknesses T1 and T2. In some embodiments, thickness T1of dummy dies 30 is smaller than the thicknesses T2 of device dies 24.The top-view area of dummy dies 30 is smaller than the top-view area ofthe resulting package 60 (FIG. 6). In some embodiments, all dummy dies30 that are placed over carrier 20 have the same size and same shape. Inalternative embodiments, dummy dies 30 have more than one shape and/orsize to fit different areas.

As also shown in FIG. 1B, after the placement of device dies 24 anddummy dies 30, molding compound 36 is filled into the gaps betweendevice dies 24 and dummy dies 30. Molding compound 36 has a CTE greaterthan the CTE of dummy dies 30. In some embodiments, the CTE of moldingcompound 36 is in the range between about 7 ppm/° C. and about 50 ppm/°C. Furthermore, the CTE of molding compound 36 may be greater than theCTE of dummy dies 30 by a difference greater than about 3 ppm/° C., orgreater than about 5 ppm/° C., although greater or smaller differencesmay also be usable. The difference in CTEs may also be in the rangebetween about 1 ppm/° C. and about 10 ppm/° C. In an exemplary moldingprocess, molding compound 36 is molded, with a top surface of moldingcompound 36 being higher than the top surfaces of device dies 24 anddummy dies 30. A curing process is then performed to cure moldingcompound 36. Next, a planarization, such as a grinding, is performed toremove excess molding compound 36, so that the top surface of moldingcompound 36 is planar. In an embodiment, the planarization is performeduntil the top surfaces of device dies 24 are exposed. The top surfacesof dummy dies 30 may or may not be exposed after the planarization.Accordingly, there may be, or may not be, a layer of molding compound 36covering dummy dies 30.

The placement of dummy dies 30 and device dies 24 is illustrated inFIGS. 2 through 4 in accordance with some embodiments. Referring to FIG.2, carrier 20 is provided. Die-attach film 22 is laminated on the topsurface of carrier 20. Dummy dies 30 are first placed in tray 38, fromwhich pick-and-place tool 40 is used to pick up dummy dies 30 one byone. Pick-and-place tool 40 places dummy dies 30 onto the desirablelocations of die-attach film 22, wherein the desirable locations aredescribed referring to FIG. 1A. During the placement, carrier 20 may beheated, and hence die-attach film 22 is accordingly heated, for example,to a temperature in the range between about 80° C. and about 150° C.Accordingly, die-attach film 22 is adhesive, and dummy dies 30 may beplaced thereon. In the embodiments in which dummy dies 30 includedifferent types having different sizes and/or different shapes, theremay be more than one tray 38, each configured to hold one type of dummydies 30. In these embodiments, pick-and-place tool 40 is configured topick-and-place dummy dies 30 from different trays.

FIG. 3 illustrates the carrier 20 and die-attach film 22 with dummy dies30 thereon. As shown in FIG. 3, dummy dies 30 are placed overlapping theperipheral region 20A of carrier 20, with the inner region 20B ofcarrier 20 not being placed with dummy dies 30. Next, as shown in FIG.4, device dies 24 are placed, for example, using the pick-and-place tool40 (FIG. 2). In alternative embodiments, device dies 24 are placed ondie-attach film 22 before the placement of dummy dies 30. After alldevice dies 24 and dummy dies 30 are placed, molding compound 36 (FIG.1B) is molded. The molding process has been discussed referring to FIG.1B. The resulting structure is shown in FIGS. 1A and 1B.

FIGS. 5 and 6 illustrate the process steps performed after the moldingof device dies 24 and dummy dies 30. Referring to FIG. 5, RedistributionLines (RDLs) 44 and dielectric layers 46 are formed to connect to thedevices 28 (such as transistors) in device dies 24. Dielectric layers 46in these embodiments may comprise polyimide, benzocyclobutene (BCB),polybenzoxazole (PBO), or the like. Alternatively, dielectric layers 46may include non-organic dielectric materials such as silicon oxide,silicon nitride, silicon carbide, silicon oxynitride, or the like.Redistribution lines 44 may be formed of aluminum, copper, nickel,tungsten, or other conductive materials. RDLs 44 have a fan-outstructure, which means that RDLs 44 extend beyond the boundaries of therespective underlying device dies 24, and hence RDLs 44 areredistributed to the areas (in the top view of FIG. 5) larger than therespective underlying device dies 24.

FIG. 5 also illustrates the formation of electrical connectors 48 inaccordance with some exemplary embodiments. The formation of electricalconnectors 48 may include forming Under-Bump Metallurgies (UBMs) 47 onRDLs 44, placing solder balls on the exposed portions of UBMs 47, andthen reflowing the solder balls. In alternative embodiments, theformation of electrical connectors 48 includes performing a plating stepto form solder regions over RDLs 44 or UBMs 47, and then reflowing thesolder regions. Electrical connectors 48 may also include metal pillars,or metal pillars and solder caps, which may also be formed throughplating. In the embodiments in which electrical connectors 48 are solderballs, the lower parts of solder balls 48 may be molded in a moldingcompound (not shown), while the upper parts of solder balls 48 areexposed. Throughout the description, the combined structure includingdevice dies 24, molding compound 36, and the overlying RDLs 44 anddielectric layers 46 is referred to as composite package 50, which maybe a composite wafer.

Next, composite package 50 is de-bonded from carrier 20. Die-attach film22 is also removed from package 50. The resulting structure is shown inFIG. 6. As a result of the de-bonding, molding compound 36 and the backsurfaces of device dies 24 and dummy dies 30 are exposed. Next,composite package 50 is further adhered to dicing tape 52, whereinelectrical connectors 48 face toward, and may contact, dicing tape 52.Composite package 50 is sawed into a plurality of packages 60, which maybe identical to each other. Each of packages 60 includes device die(s)24, and may not include any dummy die 30.

FIGS. 7 through 11 illustrate the perspective views of intermediatestages in the forming of a package structure in accordance withalternative embodiments. In these embodiments, dummy dies 30 arepre-placed in recesses of a mold, and then are transferred to adie-attach film. During the transferring, the relative positions ofdummy dies 30 are preserved. Unless specified otherwise, the referencenumerals in these embodiments (and the embodiments in FIGS. 12 and 13)represent like elements in the embodiments illustrated in FIGS. 1through 6.

Referring to FIG. 7, mold 62 is provided. Mold 62 may be a metal mold, aceramic mold, a graphite mold, a bakelite mold, or an organic mold.Recesses 64 are formed in mold 62, and extend from a planar top surfaceof mold 62 to an intermediate level of mold 62. The depths of recesses64 may be equal to, slightly greater than, or slightly smaller than, thethicknesses of dummy dies 30. The positions of recesses 64 are designedto be the same as the positions of dummy dies 30 in FIG. 1A. The sizesof recesses 64 are designed to hold dummy dies 30 therein with areasonable margin. For example, the lengths and widths of recesses 64may be greater than the respective length and widths of dummy dies 30 bya margin between about 20 μm and about 100 μm.

A plurality of dummy dies 30, which have the number greater than thenumber of recesses 64, is poured onto mold 62. Mold 62 is then vibrated,so that dummy dies 30 fall into recesses 64, with each of the recesses64 filled with one dummy die 30. Excess dummy dies 30 are then removed.The resulting structure is shown in FIG. 8.

Next, as shown in FIG. 9, vacuum suction plate 66 is moved over mold 62and dummy dies 30. Vacuum suction plate 66 includes a plurality of holes68 therein, through which the air may be vacuumed in the direction ofarrow 70. Each of holes 68 is aligned to one of dummy dies 30.Accordingly, through the vacuum, dummy dies 30 are picked up by vacuumsuction plate 66 simultaneously.

Referring to FIG. 10, vacuum suction plate 66 is moved over, and alignedto, carrier 20 and die-attach film 22. Dummy dies 30 are released ondie-attach film 22. Carrier 20 is heated, and hence die-attach film 22is sticky, so that dummy dies 30 are adhered to die-attach film 22.Through the process as illustrated in FIGS. 7 through 10, dummy dies 30are transferred to die-attach film 22, with the relative positions ofdummy dies 30 being the same as the relative positions shown in FIG. 1A.The resulting structure is shown in FIG. 11.

In subsequent processes, as shown in FIGS. 1A and 1B, device dies 24 areplaced on die-attach film 22, followed by molding device dies 22 anddummy dies 30 in molding compound 36. In alternative embodiments, devicedies 24 are placed over die-attach film 22 before the placement of dummydies 30. The process steps in FIGS. 5 and 6 may then be performed tofinish the packaging process.

FIGS. 12 and 13 illustrate the placement of dummy dies 30 in accordancewith yet alternative embodiments. The initial steps of these embodimentsare essentially the same as shown in FIGS. 7 and 8, during which dummydies 30 are placed in recesses 64 in mold 62. In these embodiments, thethickness of dummy dies 30 is greater than the depths of recesses 64, sothat dummy dies 30 protrude above the top surface of mold 62, forexample, by about 20 μm to about 50 μm.

Next, as shown in FIG. 12, carrier 20 and die-attach film 22 are placedonto mold 62, with die-attach film 22 facing toward dummy dies 30.Before die-attach film 22 touches dummy dies 30, an alignment isperformed, so that dummy dies 30 contact the desirable positions ofdie-attach film 22. Mold 62 is heated, and dummy dies 30 are alsoheated, for example, to a temperature in the range between about 80° C.and about 150° C. Accordingly, when dummy dies 30 contact die-attachfilm 22, dummy dies 30 are adhered to die-attach film 22.

Carrier 20 and die-attach film 22 are then lifted, with dummy dies 30remain to be adhered on die-attach film 22 during the lifting. Theresulting dummy dies 30 are hence placed on die-attach film 22, as shownin FIG. 13. In subsequent processes, as shown in FIGS. 1A and 1B, devicedies 24 are placed on die-attach film 22, followed by molding devicedies 24 and dummy dies 30 in molding compound 36. The process steps inFIGS. 5 and 6 may then be performed to finish the packaging process.

In the embodiments of the present disclosure, by placing dummy dies overa carrier along with the device dies, the dummy dies occupy the spacethat otherwise will be occupied by a molding compound. Since the dummydies have a CTE smaller than the CTE of the molding compound, theoverall CTE of the layer that includes the molding compound, the devicedies, and the dummy dies is reduced compared to the layer that does notinclude the dummy dies. Since the CTE of the molding compound is muchhigher than the CTE of the carrier, during the manufacturing process,the carrier and the molding compound has warpage, which affects themanufacturing process, and sometimes makes the processes (such as thephotolithography for forming RDLs and the suction of the carrier)infeasible. Accordingly, by reducing the overall CTE of the layerincluding the device dies and the molding compound through the additionof dummy dies, the warpage is reduced.

In accordance with some embodiments, a method includes placing aplurality of dummy dies over a carrier, placing a plurality of devicedies over the carrier, molding the plurality of dummy dies and theplurality of device dies in a molding compound, forming redistributionline over and electrically coupled to the device dies, and performing adie-saw to separate the device dies and the molding compound into aplurality of packages.

In accordance with other embodiments, a method includes placing aplurality of dummy dies on a die-attach film, wherein the die-attachfilm is further overlying a carrier, and placing a plurality of devicedies on the die-attach film. The plurality of dummy dies overlaps aperipheral region of the carrier, and the plurality of device diesoverlaps an inner region of the carrier. The method further includesmolding the plurality of dummy dies and the plurality of device dies ina molding compound to form a composite wafer, wherein the plurality ofdummy dies has a CTE smaller lower than a CTE of the molding compound.The method further includes forming redistribution lines over andelectrically coupled to the plurality of device dies, and demounting thecomposite wafer from the carrier, and performing a die-saw to separatethe composite wafer into a plurality of packages. Each of the pluralityof packages includes one of the device dies, and is free from dummy diestherein.

In accordance with yet other embodiments, a method includes placing aplurality of dummy dies in recesses of a mold, and picking up theplurality of dummy dies simultaneously using a vacuum suction plate. Thevacuum suction plate includes a plurality of holes, each aligned to oneof the plurality of dummy dies during the picking up. The plurality ofdummy dies picked up by the vacuum suction plate is released onto adie-attach film, wherein the die-attach film is laminated on a carrier.The plurality of dummy dies is distributed overlapping peripheralregions of the carrier, with an inner region of the carrier encircled bythe peripheral region free from overlying dummy dies. The method furtherincludes placing a plurality of device dies overlapping the inner regionof the carrier, molding the plurality of dummy dies and the plurality ofdevice dies in a molding compound to form a composite wafer, formingredistribution lines over and electrically coupled to the plurality ofdevice dies, and performing a die-saw to separate the composite waferinto a plurality of packages.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: placing a plurality of dummydies over a carrier; placing a plurality of device dies over thecarrier; molding the plurality of dummy dies and the plurality of devicedies in a molding compound; forming redistribution line over andelectrically coupled to the device dies; and performing a die-saw toseparate the device dies and the molding compound into a plurality ofpackages.
 2. The method of claim 1, wherein the plurality of packages isfree from dummy dies therein.
 3. The method of claim 1, wherein theplurality of device dies is placed overlapping an inner area of thecarrier, and wherein the plurality of dummy dies is placed overlapping aperipheral area of the carrier, with the peripheral area encircling theinner area, and wherein none of the plurality of dummy dies is placed tooverlap the inner area.
 4. The method of claim 1, wherein the placingthe plurality of dummy dies comprises picking up the plurality of dummydies one by one, and placing the plurality of dummy dies over thecarrier one by one.
 5. The method of claim 1, wherein the placing theplurality of dummy dies comprises: placing the plurality of dummy diesin recesses of a mold; and transferring the plurality of dummy dies overthe carrier, wherein relative positions of the plurality of dummy dieson the carrier are same as relative positions of the plurality of dummydies in the recesses of the mold.
 6. The method of claim 5, wherein thetransferring the plurality of dummy dies comprises: simultaneouslypicking up the plurality of dummy dies from the mold using a vacuumsuction plate; and simultaneously placing the plurality of dummy diesover the carrier.
 7. The method of claim 5, wherein the transferring theplurality of dummy dies comprises: heating the mold and plurality ofdummy dies; and contacting the plurality of dummy dies to an adhesivedie-attach film, wherein the adhesive die-attach film is laminated onthe carrier.
 8. A method comprising: placing a plurality of dummy dieson a die-attach film, wherein the die-attach film is further overlying acarrier; placing a plurality of device dies on the die-attach film,wherein the plurality of dummy dies overlaps a peripheral region of thecarrier, and wherein the plurality of device dies overlaps an innerregion of the carrier; molding the plurality of dummy dies and theplurality of device dies in a molding compound to form a compositewafer, wherein the plurality of dummy dies has a Coefficient of ThermalExpansion (CTE) smaller lower than a CTE of the molding compound;forming redistribution lines over and electrically coupled to theplurality of device dies; demounting the composite wafer from thecarrier; and performing a die-saw to separate the composite wafer into aplurality of packages, wherein each of the plurality of packagescomprises one of the device dies, and is free from dummy dies therein.9. The method of claim 8, wherein the plurality of dummy dies has sizesdifferent from each other.
 10. The method of claim 8, wherein thecarrier comprises a glass carrier, and wherein the plurality of dummydies comprises glass dies.
 11. The method of claim 8, wherein theplacing the plurality of dummy dies comprises: placing the plurality ofdummy dies in recesses of a mold; and transferring the plurality ofdummy dies onto the die-attach film, wherein relative positions of theplurality of dummy dies on the die-attach film are same as relativepositions of the plurality of dummy dies in the recesses of the mold.12. The method of claim 11, wherein the placing the plurality of dummydies in the recesses comprises: pouring the plurality of dummy dies overthe mold; and vibrating the mold and the plurality of dummy dies. 13.The method of claim 11, wherein the transferring the plurality of dummydies comprises: simultaneously picking up the plurality of dummy diesusing a vacuum suction plate; and simultaneously placing the pluralityof dummy dies over the carrier.
 14. The method of claim 11, wherein thetransferring the plurality of dummy dies comprises: heating the mold andthe plurality of dummy dies; and contacting the plurality of dummy diesto the die-attach film.
 15. A method comprising: placing a plurality ofdummy dies in recesses of a mold; picking up the plurality of dummy diessimultaneously using a vacuum suction plate, wherein the vacuum suctionplate comprises a plurality of holes, each aligned to one of theplurality of dummy dies during the picking up; releasing the pluralityof dummy dies picked up by the vacuum suction plate onto a die-attachfilm, wherein the die-attach film is laminated on a carrier, and whereinthe plurality of dummy dies is distributed overlapping peripheralregions of the carrier, with an inner region of the carrier encircled bythe peripheral region free from overlying dummy dies; placing aplurality of device dies overlapping the inner region of the carrier;molding the plurality of dummy dies and the plurality of device dies ina molding compound to form a composite wafer; forming redistributionlines over and electrically coupled to the plurality of device dies; andperforming a die-saw to separate the composite wafer into a plurality ofpackages.
 16. The method of claim 15 further comprising, during thereleasing the plurality of dummy dies, heating the die-attach film. 17.The method of claim 15, wherein the placing the plurality of dummy diesin the recesses comprises: pouring the plurality of dummy dies over themold; and vibrating the mold and the plurality of dummy dies.
 18. Themethod of claim 15, wherein the plurality of dummy dies has aCoefficient of Thermal Expansion (CTE) smaller lower than a CTE of themolding compound.
 19. The method of claim 18, wherein the plurality ofdummy dies comprises glass dies.
 20. The method of claim 18, wherein theredistribution lines extend beyond edges of the respective underlyingones of the plurality of device dies.